The standard sheathed resistance element has been around for many decades. These standard elements typically use a spiral wound resistance wire with conductor leads on both ends, surrounded by dielectric and heat transfer material and compacted to extend the thermal and dielectric capabilities and make it formable with common bending practices. One of the limitations associated with the use of a coiled resistance element which per lineal inch of heater cause the fast buildup of resistance even with heavier resistance wires which would need to be wound on a very tight coil pattern to fit into a marginally sized tubular sheath. These units are excellent choices for common heating systems that do not demand the spatial conservation or ultra low resistances and disproportional large power levels. Standard maximum power:voltage (p:v) ratios for these customary units are 2000:120 (18 amps).
Other heater elements which are used in special resistive heating situations are the single ended compacted tubular elements which help overcome some of the built in deficiencies of the customary heater element designs described above. These units use a hairpin style resistive element within a compacted tubular sheath, giving it greater flexibility and usefulness in smaller equipment or process footprints. These would not lend themselves useful in low voltage high power applications as the system would need to accommodate the use of side-by-side massive conductor legs. This would make the heater sheath too massive for space constrained operations, also the mass of the element would cause a innate thermal burden on the resistive wire causing the resistance wire to prematurely expire. Standard theoretical p:v ratio achieved by such a unit could be up to 750:120 (6.25 amps) however the likelihood of heater longevity would be difficult to ascertain without formal study.
The final notable compacted sheath style heating element we observe is the single line style heater element (seen in U.S. Pat. No. 6,456,785 to Evans). This design overcomes further the deficiencies of the single ended heater design with hairpin resistive circuit by using straight single line resistance wire further compacted with slide splice ends and small diameter conductor pins the unit steps closer to achieving greater p:v ratios up to 2500:120 (21 amps) as a standard maximum. Auxiliary cooling and specialized conductor materials are required to achieve greater ratios so that the conductor pins do not overheat and melt making the unit difficult to commercialize and produce. With the low voltages the naturally occurring oxide layers developing between the resistance wire and the slide splice create a resistive break causing the unit to lose continuity after several hours of operation.